I am a time traveller. I want to change the result of the Battle of Hastings. To achieve this, I need to learn king Harold Godwinson's (and his baronets') trust. To do this, I decide to reveal that I am from the future.
To avoid being taken as a: liar, spy or madman I need to proof that my words are true.
I decide to not reveal anything from the (known to me) future, because:
- I don't want to contaminate this timeline,
- I am uncertain how changing the result of the Battle of Hastings will influence future.
For similar reasons, I won't explain anything that I will do to prove my truthfulness. I just need to do some demonstration -- using materials available and technology known in 11th century.
I do not poses any extraordinary powers. I am not even able to travel through time -- I've appeared shortly before the Battle of Hastings as an effect of a time-accident. I have knowledge and skills as nearly every, regular 21th century university graduate would have.
The nearest sun eclipse for this area will occur too late, so using Mark Twain's idea is not an option.